Molecular sieve membranes such as zeolite membranes have been proposed for uses such as molecular separations, reactions, and combined separations and reactions. Such membranes comprise a zeolite layer, or other inorganic layer capable of molecular separations, on porous or solid substrates. The membranes may also include optional growth enhancing layers, seed layers, and reparation layers. Additionally, the zeolite layer may have a catalytic functionality present in the layer itself, in some cases inherently, or in the form of a proximately-located catalytically active material.
However, some separation, reaction, or combined separation/reaction processes cannot be performed with zeolite membranes known in the art. Some examples of such processes include cases where the catalytic functionality is in the form of catalyst distributed, sometimes with a binder, on or in to one or more layers of the membrane. Unfortunately, such membranes would be subject to catalyst attrition. Binder contamination is also a problem, especially in cases where the binder is itself catalytic.
Zeolite compositions capable of combined separation and reaction are known in the art. One example of such a composition uses conventional catalyst particles on or near the separation layer so that specific products and reactants can be selectively added or removed from the reaction zone. In other examples of such compositions, the catalytic functionality, which may be in the form of catalyst particles, is located within the zeolite layer.
It is desirable to apply compositions combining reaction and separation functionalities to processes involving staged separations. In staged separations, one or more reactants are separated from a reactant source by a zeolite or zeolite-like layer. One or more additional layers may be used to further separate the reactants, reactants and products, or products formed in or between the layers. Monolithic structures containing two or more zeolite layers in combination with catalytic functionality and capable of staged separation are not known in the art. Obstacles to forming such compositions include materials-based incompatibilities between the individual layers and between layers and particulate or layer-based catalytic functionality.
Therefore, there remains a need for membrane compositions having two or more zeolite layers and capable of separations, reactions, and combined reaction and separation wherein the catalytic functionality is either an integral part of the membrane or chemically bound to the membrane, especially when the catalytic functionality is in the form of a membrane. In particular, there is a need for a monolithic composition capable of separation, reaction, and combined reaction and separation having at least two zeolite layers in contact, the layers being composed of independently selected zeolite.